1. Field of the Invention
This invention relates to a prism optical system for an imaging system, and, more particularly, a prism system provided with partially reflecting regions which are selective reflecting mirrors capable of splitting light into different light components.
2. DESCRIPTION OF THE PRIOR ART
Generally the art of splitting light reflected from an object into different beams and especially into three primary color components has mainly been confined to the field of color television cameras. The known types of such optical systems include those that utilize dichroic mirrors, color separating mirrors, dichroic regions, or dichroic layers. It is known to use several of these mirrors or regions in order to split the original light into three primary color components. Particularly the prism systems using dichroic layers are compatible with a basically compact optical construction because they are adapted for utilization within the confines of a color television camera. Reference is made to the following United States Patents:
U.S. Pat. No. 3,922,069 issued to Kishikawa on Nov. 25, 1975 and entitled "Color Separating Prism System;" and U.S. Pat. No. 4,035,836 issued to Miyaii on July 12, 1977 and entitled "Prism Optical System for a Color Television Camera".
Both Kishikawa and Miyaji show prism systems which are adapted for utilization within a television camera. Basically both patents show systems separating light received from an optic system, which ensures that the light rays are parallel, into three color components of parallel light. The typical color television camera has three image pickup tubes each of which receives one of the color components. The placement within the camera of the pickup tubes is determined by the exit axis of the light components; therefore, the pickup tubes may be located at any appropriate location within the camera as long as they do not interfer with the function of the optic system which directs the light into the color separating prism system.
Kishikawa shows a prism system which has two prisms spaced by a third prism with dichroic layers or regions separating the intermediate prism from the other prisms. The surfaces of the intermediate prism and their associated dichroic regions are not parallel. Each of the dichroic regions reflects a different light component. Thus, two of the three light components are reflected back towards the entry surface and are reflected by the rear of the entry surface. The third light component formed is along the same optical axis as the light entering the prism system because this component is not reflected by either of the dichroic regions.
The Miyaji Patent is also adapted for utilization within a television camera system. It also three prisms of which two are separated by a third prism. Dichroic layers are situated between the middle prism and the two outer prisms. One color component is developed when the first dichroic layer reflects light onto the rear of the entry surface. The rear of the entry surface reflects that light into a pickup tube. Remaining light passes through that first dichroic layer and through the middle prism. A portion of the remaining light (the second color component) is reflected by the second dichroic layer out of the intermediate prism and into a second pickup tube. The light passing through both the first and second dichroic layers forms the third color component. The third color component passes into the third pickup tube along the same axis as the light entering the prism system. An assumption is made in both the Kishikawa and Miyaji prism systems that the pickup tubes may be placed at any location which is desirable in the camera. The space requirements are less stringent within television cameras than those, for example, on a mechanical transport which is moving documents past a read station.
These types of color separators shown in these patents are designed for a particular purpose which is to allow the reproduction of the color image by a television receiver. Color separators for this purpose are complicated by the fact that not only is the image reproduced sometime after separation but also the ultimate receiver is the human eye. Thus, the image reproduced by the television receiver must compensate for the physiological makeup of the human eye and the psychological problems inherent in the reproduction of images for viewing by human beings.
As optical character recognition (hereinafter referred to as "OCR") techniques have improved and the desirability of maximizing print contrast for mediums which may have many different colors in the background has increased. Therefore, it has been of increasing importance the OCR systems be color discriminatory and have the ability to maximize print contrast based on the separation of color. The Naval Electronics Laboratory Center has issued two annual reports (dated October 1975 and October 1976) concerning the use of different parts of the spectrum to yield an optimum contrast. The reports, however, do not go into specific color separator subsystems. Also, with the advent of new photocell devices which are more sensitive to light it is possible that the light reflected from a document can be split into several beams with each beam focused onto a different photocell device. The problem of locating a color separator or beam separator on most OCR transports is that the space between the area where the document is located with all the necessary mechanical hardware adjacent thereto for driving the document and the data lift is limited. It is highly desirable that the channels of the color or beam separator be directed in the same general direction although it is not necessary that they be directed in the same direction i.e., parallel. None of the prior construction of color or beam separators shows a design for a relatively simple prism which results in a distribution of components along axes which are generally in the same direction and is suitable for utilization on a transport.